1. Field of the Invention
This invention relates to a transducer package for high pressure applications.
2. Description of the Prior Art
Transducers using silicon crystals to measure pressure differentials are well known. Examples may be found in U.S. Pat. No. 4,291,293 issued Sept. 22, 1981 to Yamada, et. al. and U.S. Pat. No. 4,523,964 issued Jun. 18, 1985 to Wilner, et. al. Many applications for sensor transducers are known, including accelerometers, flow measurements, temperature sensors and humidity sensors.
A potential and particularly useful transducer employs an integrated circuit silicon crystal with a micromachined diaphragm. The silicon diaphragm deflects upon the application of pressure which results in an electrical output that is proportional to the input pressure. A silicon pressure sensor enclosure that can be interfaced with most harsh media usually consists of a stainless steel housing structure and utilizes silicone fluids or gels to transfer pressure from a stainless steel protective diaphragm to the crystal.
An existing problem with such transducers is long-term drift and/or impedance of the sensor performance due to mounting the silicon crystal onto a supporting substrate. Changes in sensor characteristics are also ascribed to the high coefficients of thermal expansion of silicone fluids and gels.
The prevalent use of silicones as the pressure transfer fluid is attributed to low toxicity, general inertness and their ability to shield the vulnerable silicon crystal against external contamination. Generally, a great deal of effort is directed toward minimizing the fluid fill area between the protective diaphragm and the silicon transducer to compensate for the high coefficient of thermal expansion of the silicones.
One means of packaging sensors at the silicon pressure transducer chip level is known as electrostatic bonding which allows the silicon chip to be bonded to a glass support chip. The electrostatic bond is formed using a combination of heat and voltage. The silicon and glass are placed in contact and are heated to a temperature of, typically, 450.degree. C. As voltage is then applied, mobile sodium ions in the glass drift toward the cathode and away from the glass-silicon interface. A large fraction of the applied voltage is dropped across this interface and the resulting electric field pulls the glass and silicon into contact. If the bonding procedure is correctly carried out, the bond strength to silicon is above the silicon fracture limit.
However, both internal and external stresses that develop during the electrostatic bonding procedure tend to evoke stresses on the pressure transducer causing long-term drift or mechanical damage. The quality of the bond is also dependent on the surface roughness of the silicon and glass materials being joined, and the atmosphere in which the processing occurs.
Glass sealing is also employed in packaging pressure transducers. An important criterion in any glass sealing application involves the thermal expansion rates of the silicon chip and the glass being joined together. For example, at high temperatures, the glass deforms by viscous flow, compensating for the thermal expansion difference of the silicon pressure transducer. However, when the glass and the silicon cool, each material shrinks along its own thermal expansion curve. Viscous deformation stops and stresses are produced at the interface between the two materials. Other factors such as the cooling rate, the annealing procedure, and the geometric design of the seal also affect the level of stress induced at the interface.